In a crystalline or polycrystalline thin film transistor (hereinafter called a “TFT”) to be formed on a glass base plate for a display panel such as a liquid crystal display panel, miniaturization of a TFT is under study in order to upgrade operation performance.
A glass base plate on which a TFT is formed is remarkably varied in board thickness within its surface in comparison with a semiconductor wafer. There exists such a variation in board thickness of the glass base plate as approximately 9 μm per area of a square with a side of 100 mm (about 9 μm/100 mm2) (due to surface roughness or distribution of board thickness of the base plate itself).
When forming a TFT on such a glass base plate as mentioned above, exposure of a photoresist on the glass base plate requires a depth of focus deeper than exposure for producing a semiconductor device on a semiconductor wafer. In other words, when producing a TFT by miniaturizing, it is necessary to upgrade resolution of an exposure equipment to be used then and also not to shallow the depth of focus. The resolution of the exposure equipment is around 5 μm and 3 μm at present. In this exposure equipment, should the resolution be raised from the above-mentioned value, the depth of focus becomes shallow.
When the resolution becomes high, e.g., over 0.8 μm or so, the depth of focus becomes shallower than approximately 9 μm/100 mm2, which is a variation in glass plate thickness for forming a TFT, so that it becomes impossible to form a desired resist pattern by using the photoresist on the glass base plate. In the case of exposing a photoresist on a glass plate having a great board thickness variation, difference of elevation of the photoresist surface is so great that a deeper focus is required by an optical system of the exposure equipment. An exposure process in production of a TFT includes a formation process of contact holes requiring a deep depth of focus. With the film thickness of the photoresist in this case, usually the depth of focus at the time of exposure becomes shallower than in the case of a line system pattern, and it is necessary to devise not to shallow it.
For instance, the depth of focus for forming contact holes of 0.5 μm2 is ±0.9 μm, which is very shallow. If such contact holes are formed for production of a TFT that is to be formed on a glass base plate by an exposure equipment, an area off the depth of focus is so broad that the photoresist stays on the bottom of the contact holes and prevents opening of through holes, so that there is no contacting a source area and a drain area, for example.
Also, the contact holes of an exposure mask when forming a TFT generally have a square shape. By exposing a photoresist through such a mask, the contact holes for the TFT formed on the photoresist are rounded at their corners to become substantially circular. As a result, according to the above-mentioned exposure method, it is hard to form holes penetrating the photoresist.
In an exposure art, improving a resolution and a depth of focus in an attenuated type mask (to be called an “ATT mask” herein) is described on pages 39-40 of nonpatent document 1.
Also, an art to expose a resist to be used in a machining process of a semiconductor wafer for a semiconductor device and a semiconductor thin film and the like to be formed on a glass base plate for a display panel by using an ATT mask is described in patent document 1.
Nonpatent document 1
“Ultrafine Machining Technique” issued Feb. 25, 1997, by Ohmsha.
Patent document 1
Japanese Patent Appln. Public Disclosure No. 2003-234285 Official Gazette.